1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly, to a transflective liquid crystal display and method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for a high contrast ratio.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) is classified as a transmission type and a reflection type depending on implementing an internal or external light source. The transmission type has a liquid crystal display panel, which does not emit light itself, and has a backlight as a light-illuminating section.
The backlight is disposed at the rear or one side of the panel. The amount of the light from the backlight that passes through the liquid crystal panel is controlled by the liquid crystal panel in order to implement an image display. In other words, the light from the backlight varies and displays images according to the arrangement of the liquid crystal molecules. However, the backlight of the transmission type LCD consumes 50% or more of the total power consumed by the LCD device. Providing a backlight therefore increases power consumption.
In order to overcome the above problem, a reflection type LCD has been selected for portable information apparatuses that are often used outdoors or carried with users. Such a reflection type LCD is provided with a reflector formed on one of a pair of substrates. Thus, ambient light is reflected from the surface of the reflector. The reflection type LCD using the reflection of ambient light is disadvantageous in that a visibility of the display is extremely poor when surrounding environment is dark.
In order to overcome the above problems, a construction which realizes both a transmissive mode display and a reflective mode display in one liquid crystal display device has been proposed. This is so called a transflective liquid crystal display device. The transflective liquid crystal display (LCD) device alternatively acts as a transmissive LCD device and a reflective LCD device. Due to the fact that a transflective LCD device can make use of both internal and external light sources, it can be operated in bright ambient light as well as has a low power consumption.
FIG. 1 shows a typical transflective liquid crystal display (LCD) device 11. The transflective LCD device 11 includes upper and lower substrates 15 and 21 with an interposed liquid crystal 23. The upper and lower substrates 15 and 21 are sometimes respectively referred to as a color filter substrate and an array substrate.
On the surface facing into the lower substrate 21, the upper substrate 15 includes a black matrix 16 and a color filter layer 17. The color filter layer 17 includes a matrix array of red (R), green (G), and blue (B) color filters that are formed, such that each color filter is divided by the black matrix 16. The upper substrate 15 also includes a common electrode 13 over the color filter layer 17 and the black matrix 16.
On the surface facing into the upper substrate 15, the lower substrate 21 includes an array of thin film transistors (designated as TFT “T” in FIG. 1) that act as switching devices. The array of thin film transistors is formed to correspond to the matrix of color filters. A plurality of gate and data lines 25 and 27 are positioned and crossed over each other. A TFT is located near at each crossing portion of the gate and data lines 25 and 27. The lower substrate 21 also includes a plurality of pixel electrodes 19 in the area between the gate and data lines 25 and 27. Such an area is often referred to as pixel regions “P”, as shown in FIG. 1.
Each pixel electrode 19 includes a transparent portion 19a and a reflective portion 19b. The transparent portion 19a is usually formed of a transparent conductive material having good light transmissivity, such as indium tin oxide (ITO). Alternatively, the transparent portion 19a may be a hole. Moreover, a conductive metallic material having a superior light reflectivity is used for the reflective portion 19b. 
FIG. 2, a schematic cross-sectional view of a transflective LCD device 57 illustrating an operation of such devices. For convenience, the color filters 17 (shown in FIG. 1) are not shown in FIG. 2 because it does not affect the polarization state of light. As shown in FIG. 2, the transflective LCD device 57 includes lower and upper substrates 21 and 15 and an liquid crystal layer 23 having optical anisotropy is interposed therebetween.
The upper substrate 15 includes a common electrode 13 on its surface facing into the lower substrate 21. On the other surface of the upper substrate 15, an upper quarter wave plate (QWP) 45 (often referred to as a retardation film), which has a phase difference λ/4, and an upper polarizer 55 are formed in series.
The lower substrate 21 includes a transparent electrode 50 on its surface facing into the upper substrate 15. A passivation layer 48 and a reflective electrode 19b are formed in series on the transparent electrode 50. The reflective electrode 19b and the transparent electrode 50 act together as a pixel electrode (the reference numeral 19 of FIG. 1). The passivation layer 48 and the reflective electrode 19b also have a transmitting hole 19a. 
Various configurations and structures may be implemented for the pixel electrode in the transflective LCD device. However, the passivation layer 48 should be formed between the transparent electrode 50 and the reflective electrode 19b. 
In order to form a pixel electrode, a transparent conductive material such as ITO (indium tin oxide) or IZO (indium zinc oxide) is deposited on the lower substrate 21 and then patterned into the transparent electrode 50.
Next, the passivation layer 48 is formed on the transparent electrode 50. The conductive metallic material having superior reflectivity, such as aluminum (Al) or the like, is deposited on the passivation layer 48 and then patterned to form a reflective electrode 19b. In this patterning process, the transmitting hole 19a as a transparent portion is formed at the central portion of the reflective electrode 19b. Moreover the central portion of the passivation layer 48 corresponding to the hole 19a is also patterned to expose the central portion of the transparent electrode 50.
Accordingly, the transparent electrode 50 and the reflective electrode 19b serve as a pixel electrode. Moreover, this structure makes different cell gaps “d1” and “d2” between the common electrode 13 and the pixel electrode (the reflective electrode 19b and the transparent electrode 50). “d1” denotes the first cell gap between the common electrode 13 and the reflective electrode 19b while “d2” denotes the second cell gap between the common electrode 13 and the transparent electrode 50.
On the other surface of the lower substrate 21, a lower quarter wave plate 54 and a lower polarizer 52 are formed in series. Moreover, a backlight device 41 is arranged below the lower polarizer 52.
In a homogeneous liquid crystal or twisted nematic (TN), its molecules are oriented in the vertical direction when a voltage is applied (Von=5V) and used as a liquid crystal layer 23. When an optical retardation “Δn·d1” of a first cell gap is λ/4 (λ=550 nm) and a second cell gap “d2” is twice as large as the first cell gap “d1” as described by equations (1) and (2), an optical retardation “Δn·d2” of the second cell gap “d2” is shown in equation (3).Δn·d1=λ/4  (1)d2≅2d1  (2)∴Δn·d2≅λ/2  (3)
In the above equations, Δn is birefringence, d1 denotes the first cell gap between the reflective electrode and the common electrode, d2 denotes the second cell gap between the transparent electrode and the common electrode. λ is the wavelength of the light, and λ/4 is a phase shift value of the light when the light passes through a reflective portion of the liquid crystal layer 23 between the common electrode 13 and the reflective electrode 19b at once. λ/2 is a phase shift value of the light when the light passes through a transparent portion of the liquid crystal layer between the common electrode 13 and the transparent electrode 50 at once.
Accordingly, the optical retardation “Δn·d2” of the second cell gap “d2”, as shown by equation (3), is λ/2 (λ=550 nm). In the reflective mode, the ambient light passes through the liquid crystal layer 23 twice, i.e., as the ambient light is reflected by the reflective electrode 19b. 
As mentioned above, since different cell gaps (the transparent portion and the reflective portion) are formed in the liquid crystal layer 23, there is no difference in the optical retardation of light passing both through the transparent portion and through the reflective portion.
FIG. 3 shows a liquid crystal orientation in cases that the voltage is applied and not applied. As shown, molecules of the liquid crystal layer 23 are arranged in the horizontal direction along the upper and lower substrates 13 and 21 when the voltage is not applied. On the other hand, the molecules are arranged in the vertical direction perpendicular to the upper and lower substrates 13 and 21 when the voltage is applied. However, in the ON-state, the molecules close to the upper and lower substrate 13 and 21 are not oriented properly because of an anchoring energy generated between the liquid crystal molecules and each substrate.
Therefore, the liquid crystal layer 23 derives characteristics of birefringence because the liquid crystal molecules are not properly oriented. Namely, a residual optical phase retardation can exist because of unchanged orientation or alignment of some of the liquid crystal molecules that are close to the upper and lower substrates 13 and 21. These cause the light leakage in a dark state of the LCD device.
In general, in case of the TN liquid crystal that has a twisted angle of 90°, molecules detached from the upper and lower substrates are mostly arranged perpendicular to the pair of substrates when the voltage is applied since these molecules are not affected from the anchoring energy. Moreover, the molecules close to the pair of substrates are not arranged in the vertical direction. Thus, the orientation direction of the TN liquid crystal molecules close to the upper substrate are arranged perpendicular to that of the molecules close to the lower substrate. As a result, an optical effect of the TN liquid crystal is offset each other.
However, in case of the homogeneous liquid crystal that has a twisted angle of 0° as shown in FIG. 3, these molecules close to the upper and lower substrates 13 and 21 affect the optical effect of the liquid crystal layer 23. This is because an orientation direction of the molecules located close to the upper substrate 13 are parallel to that of the molecules around the lower substrate 23.
Therefore, a light leakage occurs in the dark state of the LCD device when the upper retardation film (the reference numeral 45 of FIG. 2) and the lower retardation film (the reference numeral 54 of FIG. 2) have the same phase difference value. In addition, a contrast ratio of the transflective LCD device is deteriorated by the light leakage.
FIG. 4 is a simplified cross-sectional view in order to calculate a phase retardation value of the above-mentioned homogeneous liquid crystal. As shown, the upper polarizer 55 and the lower polarizer 52 are facing into each other. The liquid crystal layer 23 that has the optical retardation λ/2 is interposed between the pair of polarizers 55 and 52. Thereafter, a transmittance is measured by a simulator such as an LCD master.
FIG. 5 is a graph illustrating a transmittance when a voltage is applied to the transflective LCD device of FIG. 4. When the voltage is applied, i.e., the TFT is turned ON, the transmittance should be ideally zero (i.e., T=0). However, the transmittance results in 0.038 (i.e., T=0.038) in experiment. Moreover, the transmittance can be calculated by the following equation (4).
                    T        =                              Sin            2                    ⁢          2          ⁢          ϕ          ⁢                                          ⁢                                    Sin              2                        ⁡                          [                                                                    π                    ·                    Δ                                    ⁢                                                                          ⁢                                      n                    ·                    d                                                  λ                            ]                                                          (        4        )            
In equation (4), “T” denotes a transmittance, “Δn·d” denotes an optical retardation, λ denotes a wavelength of light, φ denotes an angle between a tranmissive axis of the polarizer and an optical axis of the liquid crystal layer. From the above equation (4), the optical retardation “Δn·d” is 34 nm (i.e., Δn·d=34 nm), when λ is 550 nm and φ is 45 degrees.
Accordingly, the light passing through the liquid crystal layer, which includes a homogeneous liquid crystal, has an optical retardation when a voltage is applied to the related art transflective LCD device. Thus, a complete dark state can not be achieved because of the light leakage described above.